1. Technical Field
The present invention relates to a rotational position measuring device such as a resolver for measuring the rotational position or angle based on a sine wave signal and a cosine wave signal generated according to the rotational position of a rotating shaft.
2. Description of Related Art
The resolver or the rotational position measuring device of an amplitude modulation type is used in, e.g. the detection of a rotational position of a rotor of a brushless motor, and is often used by being attached to a motor passing shaft. The principle of the resolver is substantially the same as the principle of a transformer, and the resolver is configured by a primary side coil (rotor) and two secondary side coils (stator) spaced apart by 90 degrees from each other.
When an AC voltage is applied to the primary side coil, voltages are also generated at the two secondary side coils, where the amplitudes of respective voltages outputted to the secondary side coils are V sin θ and V cos θ (V is constant), θ being a rotor angle, and the rotor angle θ can be obtained based on V sin θ and V cos θ.
In the resolver, the relational expression(Vsin θ)2+(Vcos θ)2=V2 (substantially constant)is always satisfied for the output signals V sin θ and V cos θ. Generally, the failure determination is made based on whether or not the right-hand side of the expression is within a defined range as in a displacement measuring device disclosed in Japanese Patent Application Laid-Open No. 06-147922 (1994).